The present invention relates to a metallic thin film recording medium and has for its object to improve not only its magnetic properties and resistance-to-corrosion but also its productivity.
There has been an increasing demand for high density magnetic recording, and the intensive research and development has been made in order to provide the metallic thin film magnetic recording media which can satisfy the demand for high-density recording. The processes for manufacture of metallic thin film magnetic recording media (to be referred interchangeably as "magnetic thin film" in this specification) can be divided in general into two types. One type is the wet type in which the magnetic thin film is deposited on the substrate by the chemical plating processes while the other is of the dry type using the sputtering, ion-plating or vacuum evaporation techniques. The dry process is advantageous over the wet process in that reproducibility of the magnetic properties of magnetic thin films is superior and the magnetic thin films are uniform both in longitudinal and width directions. In addition, in contrast to the wet process, the dry process can eliminate the pollution problems caused by the disposal of waste solution. Thus, the dry process has been long considered very advantageous in mass production of magnetic recording media.
The inventors made extensive studies and experiments on processes for forming ferromagnetic thin films over non-magnetic substrates of glass, metals or plastics by the sputtering, ion-plating or vacuum evaporation techniques and evaluated various characteristics such as the electric-magnetic conversion characteristics, applicability and so on which in turn determine the essential characteristics of the magnetic recording media. Then, the inventors found out that there exist some very serious problems which must be solved in order to obtain the magnetic recording media having satisfactory characteristics.
Firstly, in order to satisfy the demand for high density recording, the magnetic thin film must have a high coercive force, preferably of the order of 1000 Oe, as well as a high saturation magnetization. One of the most common methods for attaining a high coercive force is to incline a substrate at an angle to the direction of incidence of vapor when a magnetic thin film is formed. However, the inventors found out that when an evaporant is Ni, a magnetic thin film having as high a coercive force as 1000 Oe cannot be obtained at all at any angle of vapor incidence. In addition, it was also found out that in the case of an evaporant of Fe or Co, a magnetic thin film having a desired value of coercive force cannot be formed unless the angle of vapor coincidence is greater than 80.degree.. At the angle of 80.degree., the vapor impinges against a substrate almost in parallel therewith so that the deposition rate becomes extremely slow. As a result, the exposure time must be increased with a resultant decrease in productivity and subsequent increase in cost which offers serious obstruction to the mass production on a commercial scale. Observations on the magnetic thin films deposited at such high angle of vapor incidence show that the packing factor drops by about 0.2 to 0.3 with the resultant decrease in magnetic density and subsequent failure in improving the recording density.
Secondly, the resistance-to-corrosion of the metallic thin film magnetic recording media is in general inferior to that of the prior art magnetic recording media of the type in which some magnetic oxide powders are dispersed in a plastic binder and coated onto the plastic substrate.
As a result of extensive studies and experiments, the inventors found out (I) that Ni has magnetic properties which are not satisfactory, but exhibits high resistance-to-corrosion and (II) that in the case of Co or Fe, satisfactory magnetic properties can be attained at a high angle of vapor incidence as described elsewhere, but Co or Fe has low resistance-to-corrosion. When a Co or Fe magnetic thin film is left in the atmosphere at a high humidity, spot-like corrosion is observed. As time elapses, the corrosion is further propagated and finally the thin film is separated from the substrate. Obviously, magnetic recording and reproduction is impossible with such magnetic media. In short, such magnetic media cannot be used in practice. Thus, separation of the thin film due to corrosion also offers serious obstacles to mass production.